1. Field of the Invention
The invention relates to a catalytic reactor bed arrangement comprising, in a specified distribution, a plurality of catalysts in one or more fixed bed reactors and a process using the same for conversion of methanol to formaldehyde. More particularly, the invention relates to (1) a catalytic reactor bed comprising, in a specified distribution, a first metal-molybdate catalyst and a second metal-molybdate catalyst, provided in one or more fixed bed reactors, and (2) a process using the same for oxidizing methanol to formaldehyde.
2. Description of the Related Art
The formation of formaldehyde involves the dehydrogenation and oxidation of methanol. One approach for converting methanol to formaldehyde involves oxidizing methanol over a silver catalyst. See, for example, U.S. Pat. Nos. 4,080,383; 3,994,977; 3,987,107; 4,584,412; 4,343,954 and 4,343,954. Typically, methanol oxidation to formaldehyde over a silver catalyst is carried out in an oxygen lean environment. One problem associated with silver catalyzed methanol oxidation is methanol leakage i.e., high amounts of unconverted methanol.
Accordingly, improved processes for oxidizing methanol to formaldehyde have been developed. These processes use a methanol/air mixture (e.g., a reactant gas feed stream of methanol, excess air and an inert carrier gas) introduced over an iron-molybdate/molybdenum trioxide catalyst. See, for example, U.S. Pat. Nos. 3,983,073 (conversion of methanol to formaldehyde using Fe2(MoO4)3 and MoO3 having a molar ratio of Mo/Fe from 1.5 to 1.7 and a degree of crystallinity of at least 90%); 3,978,136 (process for the conversion of methanol to formaldehyde with a MoO3/Fe2O3/TiO2 catalyst wherein the MoO3:Fe2O3 weight ratio is between 1:1 to 10:1 and TiO2 is present between 1 to 90 weight % of total oxides); 3,975,302 (a supported iron oxide and molybdenum trioxide catalyst wherein the atomic ratio of Mo/Fe is from 1.5 to 5); 3,846,341 (a shaped and optionally supported iron molybdate type catalyst having high mechanical strength made by reacting ammonium molybdate and ferric molybdate); 3,716,497 (an optionally shaped iron molybdate type catalyst made by admixing with NH4+Axe2x88x92); 4,829,042 (high mechanical strength catalyst of Fe2(MoO4)3 and MoO3 together with non-sintered Fe2O3); 4,024,074 (interaction product of Fe2(MoO4)3, MoO3 and bismuth oxide for catalyzing oxidation of methanol to formaldehyde); 4,181,629 (supported catalyst of iron oxide and molybdenum oxide on silica, alumina and the like); 4,421,938 (a supported catalyst of at least two oxides of Mo, Ni, Fe and the like); and 5,217,936 (a catalyst of a monolithic, inert carrier and oxides of molybdenum, iron and the like).
In comparison to the silver catalyzed processes, iron-molybdatelmolybdenum trioxide catalyzed processes produce higher yields of formaldehyde. Iron-molybdate, Fe2(MoO4)3, in combination with molybdenum trioxide, MoO3, constitute the metal oxide phases of exemplary commercially available metal oxide catalysts suitable for oxidizing methanol to formaldehyde. During the oxidation of methanol to formaldehyde, the Fe2(MoO4)3MoO3 catalyst can be generated in situ from physical mixtures of pure molybdenum trioxide, MoO3, and ferric oxide, Fe2O3. See copending patent application designated by attorney docket no. 00242.72876 and Provisional Application No. 60/081,950 entitled xe2x80x9cIn Situ Formation of Metal Molybdate Catalystsxe2x80x9d of Wachs et al., filed on Apr. 15, 1998. The molar ratio MoO3/Fe2O3 of these catalysts may be varied. Typically, such catalysts used in industrial and commercial applications contain an excess of MoO3. Thus, for example, the molar ratio MoO3/Fe2O3 may vary from 1.5/1 to 12/1 or more. Excess MoO3 is provided to ensure that sufficient amounts of Fe2(MoO4)3 are formed in situ (from the mixture of Fe2O3 and MoO3) for efficiently oxidizing methanol to formaldehyde in high yields.
Unfortunately, the use of excess MoO3 in conjunction with Fe2O3 or other metal oxides and/or metal molybdates is problematic. Oxidizing methanol to formaldehyde using a metal molybdate/molybdenum trioxide type catalyst, e.g., Fe2(MoO4)3/MoO3, is a highly exothermic process. The heat released during the oxidation reaction increases the catalyst and/or the fixed bed reactor temperature producing xe2x80x9chot spotsxe2x80x9d on the catalyst surface. These hot spots reach temperatures high enough to volatilize the Mo/MoO3 species present within metal molybdate/molybdenum trioxide type catalysts. Thus, Mo/MoO3 is sublimed from the hot spots so formed.
The sublimed Mo/MoO3 species migrate downstream (e.g., within an exemplary fixed bed reactor housing the catalyst) towards cooler regions of the fixed bed reactor or the like. Typically, the downstream migration of sublimed Mo/MoO3 species is facilitated by the incoming flow of the reactant gas feed stream containing, for example, methanol, air, and an optional inert carrier gas fed into the inlet end of a fixed bed reactor. The migrated Mo/MoO3 species crystallize in the cooler downstream regions of the fixed bed reactor, for example, in the form of MoO3 crystalline needles. Over time, the needle formation accumulates and ultimately obstructs the flow of the reactant gas feed stream through the fixed bed reactor. Thus, build up of MoO3 crystals/needles in the downstream region causes a substantial pressure drop in the reactant gas feed stream flow rate as the reactant gas feed stream is directed downstream. This pressure drop impedes the efficient oxidation of methanol to formaldehyde. See, for example, U.S. Pat. Nos. 3,983,073 (col. 1, lines 35-52); and 4,024,074 (col. 1, lines 60-68); and U.K. Patent No. 1,463,174 (page 1, col. 2, lines 49-59) describing the aforementioned volatility problem.
Often, the MoO3 needle formation that occurs in the downstream region of the fixed bed reactor is so excessive that the reactor must be shut down, the needles cleaned out, and fresh catalyst charged therein. These steps unnecessarily increase the time, cost, inefficiency and/or complexity of operating a fixed bed reactor or the like for oxidizing methanol to formaldehyde.
Accordingly, there is a need to provide a catalytic reactor bed arrangement comprising, in a specified distribution, a plurality of catalysts within one or more fixed bed reactors and a process using the same that substantially alleviates, and/or eliminates the aforementioned crystallization problems associated with metal molybdate catalysts containing volatile Mo/MoO3 species.
Further, (1) silver catalysts, (2) supported catalysts such as those containing silicon dioxide, titanium dioxide, non-sintered Fe2O3, bismuth interaction products, silica, and/or alumina, (3) high surface area solid supported catalysts, (4) catalysts containing zinc, zinc carbonates and/or indium, (5) catalysts on inert carriers of fibrous carrier material such as fibrous sheets of silica or monolithic inert carriers, (6) shaped catalysts, and (7) the like are often prohibitively expensive to use. Accordingly, there remains a need for a catalytic bed reactor arrangement (containing a specified distribution of a plurality of methanol oxidation catalysts) and a method using the same suitable for cost effectively oxidizing methanol to formaldehyde which is free or substantially free of one or more of (1) silicon dioxide, (2) titanium dioxide, (3) non-sintered Fe2O3, (4) interaction products of Fe2(MoO4)3, and MoO3, and bismuth, (5) silica, (6) alumina, (7) supported catalysts, (8) shaped catalysts for increasing mechanical strength, (9) catalysts containing Zn(CO3). 3Zn(OH),2 In(NO3)3. 3H2O or one or more of the compounds listed in U.S. Pat. No. 4,421,938, (10) a fibrous carrier material such as silica, (11) monolithic inert materials or (12) the like.
It is therefore an object of the invention to provide a catalytic reactor bed arrangement of two or more catalysts, in a specified distribution, within one or more fixed bed reactors and a process using the same for converting methanol to formaldehyde that alleviates and/or eliminates one or more of the above mentioned problems associated with the volatility of Mo/MoO3.
It has been surprisingly discovered that use of a substantially pure metal molybdate catalyst (e.g., essentially free of volatile Mo/MoO3 species) distributed in an upstream region of one or more fixed bed reactors together with a conventional type catalyst within the downstream region of the fixed bed reactor provides a high selectivity (e.g., nearly 90-100% ) and/or high conversion % (e.g., at least 85-95%) for oxidizing methanol to formaldehyde while eliminating and/or substantially alleviating the above mentioned volatility problems associated with Mo/MoO3.
According to one aspect of the invention, oxidation of methanol to formaldehyde is achieved by the exemplary process described below. The process comprises the steps of:
(a) providing at least one fixed bed reactor having an inlet end, an upstream region, a downstream region, and an outlet end, wherein the fixed bed reactor comprises a catalytic reactor bed comprising a first metal molybdate catalyst in the upstream region and a second metal molybdate catalyst in the downstream region, and wherein the upstream region is essentially free of a volatile Mo/MoO3 species;
(b) introducing a reactant gas feed stream comprising methanol into the inlet end; and
(c) contacting and oxidizing the methanol to formaldehyde with the fist metal molybdate catalyst to yield a partially oxidized reactant gas feed stream potentially containing residual methanol; and
(d) then contacting and oxidizing the residual methanol to formaldehyde with the second metal molybdate catalyst to yield a product gas stream.
According to another aspect of the invention, an exemplary catalytic reactor bed comprises a first metal molybdate catalyst in an upstream region and a second metal molybdate catalyst in a downstream region of the fixed bed reactor, respectively. The first metal molybdate catalyst must be essentially free of a volatile species of Mo/MoO3 sufficient to alleviate and/or eliminate a substantial pressure drop of the reactant gas feed stream (comprising methanol) as it flows through the fixed bed reactor.